It is known that, despite certification complying with the regulations, aircraft, in particular transport planes, may encounter situations which significantly degrade the aerodynamics, without the crew being aware thereof. Such a situation may give rise to a surprise effect which may be the cause of an unsuitable reaction from the crew, all the more so since, when the degradation of the aerodynamic performance becomes significant, the flight qualities are modified and the control of the aircraft becomes much more difficult.
The causes of degradation of performance that are best known are in particular icing, the absence of deicing on the ground of an iced-up airframe, snow, freezing rain, the application of deicing or anti-icing fluid, the picking up of insects on leading edges, the loss of a part of a leading edge or of a panel of the flying surface.
When an aircraft is confronted with one of the aforesaid situations causing a degradation of its performance, its resistance to progress through the air increases and the drag grows. In such a case, if the power is not modified, the aircraft loses speed while it is holding altitude, or the rate of climb drops when it is holding speed, which may of course become very dangerous and is not acceptable.